Field of the Invention
The present disclosure relates, generally, to turbomachines and, more particularly, to a stationary vane arrangement for a turbomachine adapted for reducing rotor blade and/or disk excitation by homogenizing the gas flow stream, both for flow and acoustic pulsations, as well as reducing the effect of vortices shed-off the stationary vanes.
Description of the Related Art
Turbomachines, such as centrifugal flow compressors, axial flow compressors, and turbines may be utilized in various industries. Centrifugal flow compressors and turbines, in particular, have a widespread use in power stations, jet engine applications, gas turbines, and automotive applications. Centrifugal flow compressors and turbines are also commonly used in large-scale industrial applications, such as air separation plants and hot gas expanders used in the oil refinery industry. Centrifugal compressors are further used in large-scale industrial applications, such as refineries and chemical plants.
With reference to FIG. 1, a multi-stage, centrifugal-flow turbomachine 10 is illustrated in accordance with a conventional design. In some applications, a single stage may be utilized. Such turbomachine 10 generally includes a shaft 20 rotatably supported within a housing 30 by a pair of bearings 40. Turbomachine 10 shown in FIG. 1 includes a plurality of stages to progressively increase the fluid pressure of the working fluid. Each stage is successively arranged along the longitudinal axis of turbomachine 10 and all stages may or may not have similar components operating on a same principle.
With continuing reference to FIG. 1, an impeller 50 includes a plurality of rotating blades 60 circumferentially arranged and attached to an impeller hub 70 which is in turn attached to shaft 20. Blades 60 may be optionally attached to a cover disk 65. A plurality of impellers 50 may be spaced apart in multiple stages along the axial length of shaft 20. Rotating blades 60 are fixedly coupled to impeller hub 70 such that rotating blades 60 along with impeller hub 70 rotate with the rotation of shaft 20. Rotating blades 60 rotate downstream of a plurality of stationary vanes or stators 80 attached to a stationary tubular casing. The working fluid, such as a gas mixture, enters and exits turbomachine 10 in the axial direction of shaft 20. Energy from the working fluid causes a relative motion of rotating blades 60 with respect to stators 80. In a centrifugal compressor, the cross-sectional area between rotating blades 60 within impeller 50 decreases from an inlet end to a discharge end, such that the working fluid is compressed as it passes across impeller 50.
Referring to FIG. 2, working fluid, such as a gas mixture, moves from an inlet end 90 to an outlet end 100 of turbomachine 10. A row of stators 80 provided at inlet end 90 channels the working fluid into a row of rotating blades 60 provided at outlet end 100 of turbomachine 10. Stators 80 extend within the casing for channeling the working fluid to rotating blades 60. Stators 80 are spaced apart circumferentially with equal spacing between individual struts around the perimeter of the casing. A diffuser 110 is provided at the outlet of rotating blades 60 for homogenizing the fluid flow coming off rotating blades 60. Diffuser 110 optionally has a plurality of diffuser vanes 120 extending within a casing. Diffuser blades 120 are spaced apart circumferentially with equal spacing between individual diffuser blades 120 around the perimeter of the diffuser casing. In a multi-stage turbomachine 10, a plurality of return channel vanes 125 are provided at outlet end 100 of turbomachine 10 for channeling the working fluid to rotating blades 60 of the next successive stage. In such embodiment, the return channel vanes 125 provide the function of stators 80 from the first stage of turbomachine 10. The last impeller in a multi-stage turbomachine typically only has a diffuser, which may be provided with or without the diffuser vanes. The last diffuser channels the flow of working fluid to a discharge casing (volute) having an exit flange for connecting to the discharge pipe. In a single-stage embodiment, turbomachine 10 includes stators 80 at inlet end 90 and diffuser 110 at outlet end 100.
With reference to FIG. 3, a schematic view of a plurality of stators 80 is illustrated. Each stator 80 has a pair of opposing longitudinal surfaces 130a, 130b oriented substantially parallel to each other. Stators 80 are desirably oriented at a same angle with respect to a longitudinal axis of turbomachine 10. Each stator 80 has a trailing edge 140 provided at its downstream end and a leading edge 150 provided at its upstream end. Trailing edge 140 of each stator 80 is shaped identically to trailing edge 140 of an adjacent stator 80. For example, trailing edges 140 may have a pointed profile ending in a rounded point. Similarly, leading edges 150 of each stator 80 may have shapes that corresponds to trailing edges 140. Leading edges 150 of each diffuser blade 120 (not shown) are desirably formed identical to trailing edges 140. For example, similar to trailing edges 140 of stator 80, leading edges of diffuser blades 120 may have a pointed profile ending in a rounded point.
An important concern in designing turbomachines is controlling the vibration of the rotating blades and the hub throughout the operating range of the turbomachine. Rotating blades and disks in turbomachinery are excited into resonant vibrations by a) upstream stator strut and/or vane wakes and potential flow interaction with downstream struts and vanes, b) other inhomogeneities in the flow stream formed by non-uniform circumferential pressure distribution, c) acoustic pulsations either at rotating blade passing frequency and/or d) vortex shedding from stationary vanes, in turn causing coincident acoustic resonance of the gas within the casing. For example, Tyler/Sofrin modes may occur due to sound waves at blade passing frequency reflecting off vanes giving spinning modes. (Ref. Tyler, J. M., and Sofrin, T. G., 1962, “Axial Flow Compressor Noise Studies”, SAE Transactions, Vol. 70, pp. 309-332.) The acoustic pulsations reflect differently off of the stator struts set back further from the impeller and reduce the effective amplitude of the spinning modes. For example, in an impeller having 15 rotating blades and 20 stator struts, there is a 5-diameter spinning mode. If the 5-diameter structural mode is equal to 20 times the rotating speed, the blade excitation can be lowered by setting half of the stator struts downstream about one-half an acoustic wave length, as wave reflections would result in phase cancellation.
These excitations cause cyclic stress, resulting in potential high cycle fatigue and failure in impellers either at rotating blades, the hub, or the cover. The impeller components can be excited to a large amplitude when a blade modal frequency corresponds to shaft rotational frequency multiplied by the harmonic number of the flow inhomogeneity seen by blades. Typically, the number of resonances with amplitude large enough to cause high cycle fatigue is limited. Since the damage rate from fatigue occurs only if infinite endurance strength of the material is breached, a modest reduction in the vibration amplitude often will eliminate high cycle fatigue as the limiting factor for blade and disk life.
One current practice to overcome these problems is to avoid operation at the resonant frequency by changing the speed rapidly when a resonance is encountered, thereby minimizing the number of fatigue cycles that a blade accumulates during operation. If the number of vibration cycles is minimized, then blade failure is controlled by mechanisms other than downstream wakes, acoustic pulsations, flow inhomogenities, or vortex shedding. However, this practice places undesirable limits on operation of turbomachinery.
Another current approach is to reduce the spatial variations in the flow field by directly injecting air into low-velocity wakes behind obstructions (Rao, N. M., Feng, J., Burdisso, R. A, and Ng, W. F., “Active Flow Control to Reduce Fan Blade Vibration and Noise”, 5.sup.th AIAA/CEAS Aeroacoustic Conference, American Institute of Aeronautics and Astronautics, May 10-12, 1999). This approach requires the use of either air from the compressor or from an additional external air source in relatively large quantities. Use of compressor air has a detrimental impact on performance. The addition of a separate air supply adds weight and requires power. Both methods have detrimental impacts on performance. Also, wake filling does not address modal excitation due to bow waves from downstream flow obstructions.
Within the prior art, a number of approaches have been proposed for reducing vibration amplitude of rotating blades and/or providing noise abatement. U.S. Patent Application Publication No. 2007/0274826 to Kuhnel et al. discloses a diffuser for a compressor impeller. FIG. 1 of the Kuhnel et al. publication discloses a diffuser structure that includes guide blades that are each formed of two component blades. The first component blade has an inlet edge and the second component blade has an inlet edge stepped back from another inlet edge. FIG. 2 shows another embodiment wherein a third component blade is provided between component blades. The stepped inlet edges are provided for noise abatement.
U.S. Pat. No. 7,189,059 to Barton et al. discloses a compressor with an inlet shroud situated about an impeller. The shroud, as shown in FIG. 2, includes a plurality of spaced apart vanes or struts with strut tips. As shown in FIG. 6, the struts are configured to vary in thickness between a first end and the strut tip. This variation in thickness is implemented as a linear taper between the strut first ends and the strut tips to increase the natural frequencies of the struts.
U.S. Pat. No. 6,439,838 to Crall et al. describes the use of variable circumferential spacing of the vanes in an axial flow turbomachine to achieve reduced vibratory excitation.
Clark, J., “Design Strategies to Mitigate Unsteady Forcing (Preprint)”, AFRL-RZ-WP-TP-2008-2112 discusses the state of the art used for reduction of excitation to rotating blades including the use of a different number of stationary vanes in the upper and lower two halves of a machine having a horizontally split arrangement.
However, none of the prior art designs are directed to a stationary vane arrangement adapted for reducing rotor blade excitation by dehomogenizing the successive wakes within the flow stream and reducing the effect of vortices shed-off the vanes, in addition to reducing acoustic pressure pulsations and direct pressure loads on the rotating blades.